Organic compounds having a fluorescent chromophore (to be referred to as “fluorescent compounds”, hereinafter) are often used as fluorescent ink or pigment to offer a colorant for paper, fibers and resins partly because they are less expensive than fluorescent inorganic compounds.
The colorants are generally classified into two categories, dyes and pigments. The pigment is an agglomerate of molecules having a chromophore and has a large particle size whereas the dye is a chromophore-containing molecule which is dissolvable in media including solvents and resins. In general, those materials colored with pigments are inferior in transparency, lightness and color saturation to those materials colored with dyes, because the pigment colorants have a larger particle size.
Except fluorescent tags wherein chemical reaction takes place between molecules, fluorescent compounds are generally used as the pigment partly because of improved light resistance and partly because of a low solubility in low polar media.
For the above reason, when resins are colored with fluorescent compounds, relatively high polarity resins like acrylic resins and polyester resins may be colored with dyes. In the case of low polarity resins like polyolefin resins and silicone resins, pigments are inevitably used because dyes are least soluble therein. In the case of coloring with pigments, to obtain transparent resin compositions, the pigment must be finely ground to a size smaller than the wavelength of light and uniformly dispersed in the resin. This grinding/dispersing step consumes a large amount of energy and time.
One method of enhancing compatibility with low polar resins is to attach a fluorescent compound to a resin. For example, Patent Document 1 describes a silicone resin obtained by bonding a fluorescent compound to an amino silicone resin.
A number of fluorescent compounds are known as well as compounds useful as the dye. For example, fluorene compounds with an emission capability of high luminance and efficiency are useful as the luminescent material in organic electroluminescent devices as disclosed in Patent Document 2. Patent Document 3 describes that a compound having an improved solubility in organic solvents is obtainable by linking two fluorene compounds via a siloxanyl group as spacer.